Polypropylene, having superior stretch properties, can be formed into uniform thin films and has been widely used in various fields because of its superior properties. Polypropylene has also been widely used for capacitor films because of its superior electrical properties. Recently, the demand for capacitor films has been growing in the fields of household appliances and automobiles, and there is a need for further improvements in the withstand voltage of capacitor films formed of propylene.
For further improvements in the withstand voltage of capacitor films formed of propylene, various propylenes have been proposed. Among those are, for example, a propylene with increased tacticity whose isotactic pentad fraction in a boiling heptane insoluble fraction is 0.955 or more (see, for example, PTL 1), a propylene with an ash content of 40 ppm by weight or less and a chlorine content of 2 ppm by weight or less (see, for example, PTL 2), and a propylene whose racemic pentad fraction in a boiling n-heptane insoluble fraction is 0.0005 to 0.01 (see, for example, PTL 3). Also proposed are, for example, a propylene with an optimally controlled meso pentad fraction and amount of burnt residue (see, for example, PTL 4), a propylene whose isotactic or syndiotactic pentad fraction in a boiling heptane soluble fraction is 0.5 or more (see, for example, PTL 5), and a propylene produced using a specific metallocene catalyst and having a pentad fraction of 93 mole percent or more, a xylene soluble fraction content of less than 1% by weight, and recoverable contents of aluminum and chlorine of less than 25 ppm (see, for example, PTL 6).
However, only reducing the amount of burnt residue (ash content) of propylenes, improving the tacticity of propylenes, and controlling the heptane or xylene soluble fraction content have not led to a material satisfying the demanded level of withstand voltage, which has been rapidly increasing recently.